1. Field of the Invention
The present invention relates to an achromatic objective lens for a microscope having a magnification of about 50 times and a working distance at least twice as large as the focal length.
2. Description of the Related Art
In general, in an objective lens for a microscope having a magnification of about 50 times, if an attempt is made to increase the working distance, spherical aberration and chromatic aberration are likely to deteriorate. Therefore, it has been very difficult to sufficiently increase both the working distance and numerical aperture.
As a long working-distance achromatic objective lens for a microscope having a magnification of about 50 times, there is known for example the one disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-90230. However, the objective lens disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 9-90230, has a numerical aperture of about 0.45, and a satisfactory numerical aperture and size of working distance cannot be obtained.
It is an object of the present invention to provide an achromatic objective lens for a microscope having a magnification of about 50 times and a long working distance, which can increase the numerical aperture up to about 0.55.
In order to achieve the above object, the objective lens for a microscope according to the present invention comprises, in order from an object side: a first lens group comprising a meniscus positive lens with a concave surface facing the object; a second lens group including a cemented lens and having a positive refractive power; a third lens group including a cemented lens and having a positive refractive power; a fourth lens group comprising a cemented lens; and a fifth lens group comprising a cemented lens.
According to the above aspects of the invention, an achromatic objective lens for a microscope having a long working distance, excellent imaging performance and a magnification of about 50 times can be obtained.
The objective lens for a microscope may satisfy the following conditional expression:
n1 greater than 1.8 
xcexd5Nxe2x88x92xcexd5P greater than 30 
wherein:
n1: refractive index of the meniscus positive lens constituting the first lens group;
xcexd5N: Abbe""s number of a negative lens in the fifth lens group;
xcexd5P: Abbe""s number of a positive lens in the fifth lens group.
The third lens group may be a cemented lens of a double-convex positive lens, a double-concave negative lens and a positive lens, in order from the object side.
The objective lens for a microscope may satisfy the following conditional expression:
1.2 less than |r1|/d0 less than 2.2 
wherein:
d0: distance on an optical axis from an object surface to a surface of the first lens group on the side closest to the object;
r1: radius of curvature of a surface of the first lens group on the side closest to the object.
The objective lens for a microscope according to a second aspect of the present invention comprises, in order from the object side: a first lens group comprising a meniscus positive lens with a concave surface facing the object; a second lens group including a cemented lens with the cementing surface having a concave surface facing the object, and having a positive refractive power; a third lens group including a cemented lens having a positive lens, a negative meniscus lens and a positive lens in order from the object side, and having a positive refractive power; a fourth lens group comprising a cemented lens; and a fifth lens group comprising a cemented lens.
The objective lens for a microscope of the second aspect may satisfy the following conditional expression:
xcexd2P greater than 65 
xcexd3P greater than 65 
wherein:
xcexd2P: Abbe""s number of a positive lens in the second lens group;
xcexd3P: Abbe""s number of a positive lens in the third lens group.
The objective lens for a microscope of the second aspect may satisfy the following conditional expression:
1.2 less than |r1|/d0 less than 2.2
wherein:
d0: distance on an optical axis from an object surface to a surface of the first lens group on the side closest to the object;
r1: radius of curvature of a surface of the first lens group on the side closest to the object.
Moreover, the third lens group may be a cemented lens of a double-convex positive lens, a double-concave negative lens, and a positive lens, in order from the object side.
Furthermore, the objective lens for a microscope of the second aspect may satisfy the following conditional expression:
xcexd5P less than 22.
The objective lens for a microscope according to a third aspect of the present invention comprises, in order from the object side: a first lens group having a meniscus positive lens with a concave surface facing the object; a second lens group including a cemented lens with a positive lens arranged on the object side, and having a positive refractive power; a third lens group including a triple cemented lens with a double-convex positive lens arranged on the object side, and having a positive refractive power; a fourth lens group comprising a cemented lens; and a fifth lens group comprising a cemented lens.
The objective lens for a microscope of the third aspect may satisfy the following conditional expression:
xcexd2P greater than 65 
xcexd3P greater than 65 
wherein:
xcexd2P: Abbe""s number of a positive lens in the second lens group;
xcexd3P: Abbe""s number of a positive lens in the third lens group.
The objective lens for a microscope of the third aspect may satisfy the following conditional expression:
n1 greater than 1.8
xcexd5Nxe2x88x92xcexd5P greater than 30 
wherein:
n1: refractive index of the meniscus positive lens constituting the first lens group;
xcexd5N: Abbe""s number of a negative lens in the fifth lens group;
xcexd5P: Abbe""s number of a positive lens in the fifth lens group.
The objective lens for a microscope of the third aspect may satisfy the following conditional expression:
1.2 less than |r1|/d0 less than 2.2 
wherein:
d0: distance on an optical axis from an object surface to a surface of the first lens group on the side closest to the object;
r1: radius of curvature of a surface of the first lens group on the side closest to the object.
The cemented lens in the third lens group may further have a double-concave negative lens and a positive lens.
The objective lens for a microscope of the first aspect may satisfy the following conditional expression:
xcexd5P less than 22. 